Closed loop air fuel mixture control using a zirconium dioxide sensor is known in the art in which the oxygen concentration of the exhaust emissions is sensed to generate an output which is utilized to generate a signal to control the engine's air fuel ratio at the stoichiometric value. Three-way catalytic convertors are desgined to operate at the stoichiometric value to convert the emissions to harmless water and carbon dioxide. With the engine operating at stoichiometry, the noxious emissions are reduced to a minimum. However, the closed loop control system would have disadvantages in that the controlled air fuel ratio permits no increase in the engine's output power, and the engine must be operated with less fuel than is needed for heavy load running.
Therefore, an object of the present invention is to provide a closed loop air fuel ratio control system for an automotive in which closed loop control is disabled or clamped to a miniumum by sensing full throttle position to permit reduction in the air fuel ratio below the stoichiometric value to allow the engine to give more power than normal when the vehicle is driven under heavy loading or high acceleration conditions.
Another object of the invention is to provide an improved closed loop air fuel ratio control system in which secondary air is supplied to a catalytic convertor when the air fuel ratio lowers below the stoichiometric value to compensate for the reduction in air during the heavy loading period to permit the convertor to operate at the maximum efficiency.